Is EL Nino on its Last Leg?

Unless we see a major surge of westerly trade winds across the equatorial Pacific and another surge of warmth within the sea-surface temperatures across the equatorial Pacific it is highly likely we have seen the current EL Nino reach its maximum strength.  If the EL Nino has indeed reached its peak, it will have tied the EL Nino of 1997-1998 for the strongest on record since 1950 and fall short of the EL Nino of 1876-1878 (this EL Nino reached its peak strength in the winter of 1877-1878) for strongest on record.  Looking at Oceanic Nino Index (ONI) data, we can see that both the EL Nino of 1997-1998 and the current EL Nino have peaked at +2.3 degrees Celsius.  Reanalysis data had the 1876-1878 EL Nino peak at +2.48 degrees Celsius:

When measuring the sea-surface temperatures and determining sea-surface temperature anomalies (anomaly meaning how far from average) across the equatorial Pacific, the basin is broken down into 4 different ENSO regions; region 1.2, region 3, region 3.4, and region 4.  Below is a visual reference:

Looking at the anomalies (in degrees Celsius) for each of these regions dating back to the end of September we can see the trends which have occurred since the end of September.  What we can take from this is that since about mid-January or so, the trend has been for cooling of the sea-surface temperatures overall with some periods of some very minor warming:

Despite the cooling we've seen the past several weeks, looking at the current region anomalies (As of March 9th in the above chart and looking at the current sea-surface temperature map we can see the EL Nino is still going strong:

Going forward with the EL Nino one thing we can look at are the 850mb trade winds across the equatorial Pacific ocean.  Below is a time-series graph showing the progression of the trade winds (the darker shadings indicate easterly trade winds and the brighter shadings indicating either very weak easterly trade winds or westerly trade winds).  One area to focus on is about 80W to 120W.  If in this region we are seeing darker shadings, this is indicating easterly trade winds which allows for colder waters off the coast of South America to push westward into the equatorial Pacific.  This would work to weaken the EL Nino state.  If between 120E to 120W we see brighter shadings, this indicates either very weak easterlies or westerly trade winds (especially once you get into the reds and pinks) and would allow for warmer waters to push towards the coast of South America which would help maintain or even strengthen the EL Nino.  Looking below after that substantial burst of westerlies centered around the international dateline there are no indications of strong westerlies and it appears that the trade winds may even be in the beginning phases of reversing back to easterlies.  If this is the case, this is a signal the EL Nino is indeed in the weakening process:

We can also look at the depth of the sea-surface temperature anomalies across the equatorial Pacific as well to give us some insight.  In order for an EL Nino to maintain or even strengthen you want to see above-average anomalies extending deep beyond the surface of the ocean.  This slows the process of upwelling and not only keeps sea-surface temperatures warm but can allow them to further warm.  If the depth of the above-average anomalies is shallow and confined close to the surface, this would allow for upwelling and mixing of the surface waters preventing warming and also leading to cooling.  Looking at the current update on March 9th you can see the depth of the above-average anomalies is not very deep.  In fact, if this graphic was looped, you would see the depth of the above-average anomalies shrinking overtime:

  

We can also look at the depth of the 25 degree Celsius thermocline as this can give us an idea of how deep the warmer waters extend.  Looking at the plot below we can see that the depth of the 25C thermocline is very shallow and in fact if this was looped you can see the 25C thermocline begin to shift upwards towards the surface.  Also notice how the 20C thermocline is beginning to near the surface as well:

What we can take from all of this is that it appears that the current EL Nino is well on its way to a weakening state.  Unless we were to see substantial changes which is very unlikely based on historical records, the current EL Nino will only tie 1997-1998 for strongest in modern records and fall shy of the all time record.  Moving forward towards the summer and beyond there are numerous indications that the EL Nino will eventually rapidly weaken and perhaps even enter La Nina territory by the end of the summer or by next Fall, but we'll save that for another post!